Metal-Free Intermolecular C–H Borylation of N-Heterocycles at B–B Multiple Bonds

Carbene-stabilized diborynes of the form LBBL (L = NHC or CAAC) induce rapid, high yielding, intermolecular ortho-C–H borylation at N-heterocycles at room temperature. A simple pyridyldiborene is formed when an NHC-stabilized diboryne is combined with pyridine, while a CAAC-stabilized diboryne leads to activation of two pyridine molecules to give a tricyclic alkylideneborane, which can be forced to undergo a further H-shift resulting in a zwitterionic, doubly benzo-fused 1,3,2,5-diazadiborinine by heating. Use of the extended N-heteroaromatic quinoline leads to a borylmethyleneborane under mild conditions via an unprecedented boron-carbon exchange process

Metallfreie intermolekulare C-H-Borylierung von N-Heterocyclen an B-B-Mehrfachbindungen

Carbenstabilisierte Diborine der Form LBBL (L=N-heterocyclisches Carben (NHC) oder cyclisches Alkyl(amino)carben (CAAC)) induzieren bei Raumtemperatur eine schnelle, ertragreiche, intermolekulare ortho-C-H-Borylierung an N-Heterocyclen. Ein einfaches Pyridyldiboren wird gebildet, wenn ein NHC-stabilisiertes Diborin mit Pyridin kombiniert wird, während ein CAAC-stabilisiertes Diborin zur Aktivierung von zwei Pyridinmolekülen führt, um ein tricyclisches Alkylidenboran zu bilden, das durch Erhitzen zu einem zwitterionischen, zweifach benzokondensierten 1,3,2,5-Diazadiborinin mittels einer weiteren H-Verschiebung umgelagert werden kann. Die Verwendung des verlängerten N-heteroaromatischen Chinolins führt unter milden Bedingungen über einen bisher unbekannten Bor-Kohlenstoff-Austauschprozess zu einem Borylmethylenboran

Computational Revision of the Mechanism of the Thorpe Reaction

The Thorpe reaction is described as a self-condensation of nitriles in the presence of a basic catalyst producing β-enaminonitriles. We performed theoretical calculations within the Density Functional Theory (DFT) framework at the ωB97XD/def2-svpd level to explore different mechanistic proposals when propionitrile is used as a reagent and sodium ethoxide (EtONa) as a catalyst. Furthermore, the influence of different solvents, such as ethanol (EtOH), tetrahydrofuran (THF), 1,2-dimethoxyethane (DME), and propionitrile (EtCN), was assessed. Finally, we also evaluated the effect of the fluorine group (-F), compared to the methyl group (-CH3), substituted in the α position of acetonitrile (MeCN). Our theoretical findings agree with different experimental reports on the Thorpe reaction

Computational Revision of the Mechanism of the Thorpe Reaction

The Thorpe reaction is described as a self-condensation of nitriles in the presence of a basic catalyst producing β-enaminonitriles. We performed theoretical calculations within the Density Functional Theory (DFT) framework at the ωB97XD/def2-svpd level to explore different mechanistic proposals when propionitrile is used as a reagent and sodium ethoxide (EtONa) as a catalyst. Furthermore, the influence of different solvents, such as ethanol (EtOH), tetrahydrofuran (THF), 1,2-dimethoxyethane (DME), and propionitrile (EtCN), was assessed. Finally, we also evaluated the effect of the fluorine group (-F), compared to the methyl group (-CH3), substituted in the α position of acetonitrile (MeCN). Our theoretical findings agree with different experimental reports on the Thorpe reaction

A Simple and Efficient Method for the Partial Synthesis of Pure (3<i>R</i>,3’<i>S</i>)-Astaxanthin from (3<i>R</i>,3’<i>R</i>,6’<i>R</i>)-Lutein and Lutein Esters via (3<i>R</i>,3’<i>S</i>)-Zeaxanthin and Theoretical Study of Their Formation Mechanisms

Carotenoids are natural compounds that have important roles in promoting and maintaining human health. Synthetic astaxanthin is a highly requested product by the aquaculture industry, but natural astaxanthin is not. Various strategies have been developed to synthesize this carotenoid. Nonetheless, these approaches have not only provided limited global yields, but its main commercial source also carries several health risks for humans. In this contribution, the one-pot base-catalyzed reaction of (3R,3&#8217;R,6&#8217;R)-lutein (1) esters has resulted in a successful isomerization process to easily obtain up to 95% meso-zeaxanthin (2), which in turn is oxidized to (3R,3&#8217;S)-astaxanthin (3) with a global yield of 68%. The same oxidation performed with UV irradiation (365 nm) for 5 min provided the highest global yield (76%). These chemical transformations have also been achieved with a significant reduction of the health risks associated with its potential human consumption. Furthermore, this is the first time only one of the configurational isomers has been obtained semisynthetically. The poorly understood formation mechanisms of these two compounds were also investigated using Density-Functional Theory (DFT) calculations. These theoretical studies revealed that the isomerization involves a base-catalyzed deprotonation at C-6&#8217;, followed by C-4&#8217; protonation, while the oxidation occurs via free radical mechanisms

The role of the metal in the dual-metal catalysed hydrophenoxylation of diphenylacetylene

Recently, the hydrophenoxylation of alkynes mediated by a gold catalyst has been shown to occur through a bimetallic process where the active species are [Au(OPh)(IPr)] and [Au((2)-alkyne)(IPr)](+). On the other hand, Cazin and co-workers have reported an improvement of the hydrophenoxylation reaction by using a combination of [Cu(OPh)(IPr)] and [Au((2)-alkyne)(IPr)](+). Herein, we performed DFT calculations to rationalize the performance of the Cu/Au bimetallic system. We have studied the rate determining step (rds) proposed by Poater et al. for the potential initial reagents PhO-[M] and alkyne-[M] species. Silver was also included in this study in order to understand the effect of the metal. Moreover, the effect of the steric hindrance on the rds was investigated with less sterically demanding ligands such as IMes, SIMes and IMe. Overall, the [Cu]/[Au] system was computed to be superior to the [Au]/[Au] system, but the [Ag]/[Au] couple exhibited the lowest energy barrier for the rds providing an enhancement of the heterometallic catalysis. We show that the rds is quite sensitive to the ligand steric hindrance

Nucleophilic addition and substitution at coordinatively saturated boron by facile 1,2-hydrogen shuttling onto a carbene donor

The reaction of [(cAAC$^{Me}$)BH$_{3}$] (cAAC$^{Me}$ = 1-(2,6-iPr$_{2}$C$_{6}$H$_{3}$)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) with a range of organolithium compounds led to the exclusive formation of the corresponding (dihydro)organoborates, Li$^{+}$[(cAAC$^{Me}$H)BH$_{2}$R]− (R = sp$^{3}$-, sp$^{2}$-, or sp-hybridised organic substituent), by migration of one boron-bound hydrogen atom to the adjacent carbene carbon of the cAAC ligand. A subsequent deprotonation/salt metathesis reaction with Me3SiCl or spontaneous LiH elimination yielded the neutral cAAC-supported mono(organo)boranes, [(cAAC$^{Me}$H)BH$_{2}$R]− (R]. Similarly the reaction of [cAAC$^{Me}$)BH$_{3}$] with a neutral donor base L resulted in adduct formation by shuttling one boron-bound hydrogen to the cAAC ligand, to generate [(cAAC$^{Me}$H)BH$_{2}$L], either irreversibly (L = cAAC$^{Me}$) or reversibly (L = pyridine). Variable-temperature NMR data and DFT calculations on [(cAAC$^{Me}$H)BH$_{2}$(cAAC$^{Me}$)] show that the hydrogen on the former carbene carbon atom exchanges rapidly with the boron-bound hydrides

Trapping of a borirane intermediate in the reductive coupling of an arylborane to a diborene

The reductive coupling of a N-heterocyclic carbene (NHC)-stabilized aryldibromoborane yields a mixture of trans- and cis-diborenes in which the aryl groups are coplanar with the diborene core. Under dilute reduction conditions two diastereomers of a borirane–borane intermediate are isolated, which upon further reduction give rise to the aforementioned diborene mixture. DFT calculations suggest a mechanism proceeding via nucleophilic attack of a dicoordinate borylene intermediate on the aryl ring and subsequent intramolecular B–B bond formation

Stable Two‐Legged Parent Piano‐Stool and Mixed Diborabenzene‐E4_{4} (E=P, As) Sandwich Complexes of Group 8

A cyclic alkyl(amino)carbene‐stabilized 1,4‐diborabenzene (DBB) ligand enables the isolation of 18‐electron two‐legged parent piano‐stool Fe$^{0}$ and Ru$^{0}$ complexes, [(η$^{6}$‐DBB)M(CO)$_{2}$], the ruthenium complex being the first of its kind to be structurally characterized. [(η$^{6}$‐DBB)Fe(CO)$_{2}$] reacts with E$_{4}$ (E=P, As) to yield mixed DBB‐cyclo‐E$_{4}$ sandwich complexes with planar E$_{4}$$^{2-}$ ligands. Computational analyses confirm the strong electron‐donating capacity of the DBB ligand and show that the E$_{4}$ ligand is bound by four equivalent Fe−P σ bonds